Jigging is a well-known process that is used to separate a material into various fractions. For example, jigging is commonly used in mineral processing and coal preparation to separate two or more mineral species from a material based on specific gravity. Generally, jigging involves particle stratification wherein the particles of a material are rearranged through alternate expansion and compaction of a fluidized bed of those particles by pulsating fluid flow that is periodically reversed. The continuously varying forces acting on the particles stratify the material into layers of particles that are arranged by increasing density from top to bottom. That is, the bed expansion and compaction effect the separation. The expansion or opening of the bed permits the particles to move away from each other. Then, during compaction or closing of the bed, hindered settling, differential acceleration and consolidation trickling cause the material to separate into layers of particles differing in specific gravity.
Stratification is generally carried out in a jig. Early jigs used a basket loaded with mixed particles that were moved up and down (jigged) in a tank of water. This agitation rearranged the particles in layers of increasing density from top to bottom. Although these jigs stratified the feed material intended to be separated, advances in jig technology were made so that a wider range of materials could be separated and higher throughput achieved.
The most common type of jig today is the fixed or flat bed jig. In a flat bed jig, a flat perforated screen plate (screen deck) supports the feed or material to be separated. A hutch compartment, from which water is pulsed to and from the bed of material supported on the screen to oscillate the particles and effect bed expansion and contraction, is formed below the screen. The water pulsations can be motivated by various means such as air pressure or piston-like plungers. The upward movement of the water through the perforated screen plate from air pressure or plunger-activated water pressure, for example, is referred to as the "pulsation" stroke, while the downward movement is termed the "suction" stroke. As the feed moves across the screen, the fraction of the feed which has a larger specific gravity and forms the bottom layer is typically removed by allowing it to simply pass through the screen. The lighter fraction flows off the downstream end of the screen deck. For a more detailed description of this process, see, for example, Leonard, III, J. W., Ed., Coal Preparation, Society for Mining, Metallurgy and Exploration, Inc., Littleton, Colo. (1991).
Centrifugal jigs were developed to amplify the gravitational forces acting on the feed and thus improve particle stratification. In centrifugal jigs, the screen is wrapped into a cylindrical configuration. The feed is introduced to a rotating plate which propels the feed to the cylindrical screen which is rotated about a vertical axis. A hutch is provided around the screen so that water can be pulsed through the screen and, thus, the feed material. As the feed material flows downward along the screen, the water pulsations effect bed expansion and compaction. The heavy fractions flow through the screen to a first collection zone, while the lighter fractions continue to flow downward to a different collection zone. The feed, which can be held against the inner surface of the screen by centrifugal force, can be periodically jigged by fluid pulses created by the interaction of a series of equi-angularly spaced fluid supply nozzles and a series of water pulse blocks leading to the hutch interior.
In order to improve the ability of jigs to handle particles having very fine sizes, the jigs described above have been provided with a bed of "ragging". The ragging, which typically comprises steel shot, is provided over the screen and permits the transverse passage of heavier particles, but impedes the flow of lighter or fine particles. In this manner, the ragging reduces the amount of light fractions that pass through the screen with the heavier fractions. However, among the drawbacks of using ragging beds is that they must be periodically cleaned so that the jig can operate efficiently. Otherwise, materials trapped in the ragging can undesirably impede the water pulsations, as well as heavy particle flow to the screen.
The expansion and compaction systems described above also are not without limitation. Generally, it is desirable to have each expansion-compaction cycle (jig cycle) repeat at a high frequency in order to effectively separate the heavy and light fractions. This is especially the case when processing extremely fine particles and/or using a centrifugal jig where the time in which the feed travels along the screen is relatively short. However, it is difficult to obtain a high pulse frequency with a water pulsing system due to a number of factors such as the fluidity of the water. That is, it is difficult to impart a tight pulsation or suction stroke to a fluid. In addition, the pulsing systems typically involve complex mechanical components including complex valving arrangements, diaphragms and so forth.
The effectiveness of jigging also is highly dependent upon the difference in specific gravity of the materials being separated. When the specific gravities are not very different, conventional jigs may not provide adequate separation.